There are several kinds of stents on the market with either balloon expandable or self expanding function. Balloon expandable stents are generally made from a material that can easily be plastically deformed into two directions. Before insertion, the stent is placed around the balloon section at the distal end of a catheter and pressed together to reduce the outer dimensions.
As soon as the stent is brought into the body in the proper axial position it can be expanded and thereby plastically deformed by pumping up the balloon. In this final position, the stent is at its largest diameter and should function to support the surrounding tissue, preventing an undesired shape change into a much smaller diameter, at least locally.
Therefore, the stent needs to have sufficient rigidity in the radial direction, but also some flexibility in the axial direction when it is in the final position. Further, the amount of material should be as small as possible and in the inner surface of the stent should not obstruct the flow through the channel (e.g., for blood) or cause too much turbulence.
Problems that generally occur with these stents are as follows: After compressing the stent to its smallest diameter around the balloon, the stent will always have some elastic spring back to a sightly larger diameter, which can cause problems when the catheter is brought into the patient""s body. In addition, the axial friction between balloon and stent can become so small that the stent slips off the catheter. Further, a larger size stent is typically a disadvantage.
A further problem is the so called recoil of these stents. This means that after expansion by the balloon pressure, the outer diameter will always become slightly smaller as soon as the balloon is deflated. This degree of recoiled can be as much as 10%, which can cause migration of the stent.
A different type of stent is made of a more or less elastically expanding structure, which has to be held on the catheter by some external means. An example of this type is a stent that is held in its constrained state by a delivery sheath, that is removed at the moment that the stent should deploy to its natural form.
Some of these stents are made of shape memory material with either superelastic behavior or temperature sensitive triggering of the expansion function.
A disadvantage of these self-expanding stents is the need for the delivery sheath, causing a larger insertion diameter. The removal of the sheath also requires a sheath retraction mechanism, which has to be activated at the proximal end.
Most stents of both types further have the disadvantage of relatively large length change during expansion and a poor hydrodynamic behavior because of the shape of the metal wires or struts.
Another disadvantage of some stents is the positive spring rate, which means that further expansion can only be achieved by higher balloon pressure.
The construction of prior stents is typically made in such a way that the external forces, working on the stent in the radial direction, merely cause bending forces on the struts or wires of the structure.
For example, a unit cell of a Palmaz-Schatz-stent, as produced by Johnson and Johnson Interventional Systems or the ACT One Coronary stent, produced by Progressive Angioplasty Systems, Inc. has in its collapsed condition a flat, rectangular shape and in its expanded condition a more or less diamond-shaped form with almost straight struts (Palmaz-Schatz) or more curved struts (ACT-One).
The shape of the unit cell of such stents is typically symmetrical with four struts each having the same cross section. In addition, the loading of the cell in the axial direction will typically cause an elastic or plastic deformation of all of the struts, resulting in an elongation of the unit cell in the axial direction. These unit cells have a positive spring rate. In stents based upon these unit cells the stability against radial pressure is merely dependent on the banding strength of the struts and their connections.
In this patent application a new type of stent is described with a unit cell, having a negative spring rate and a bistable function. Such a unit cell can also be used in other medical applications. This means that it has two configurations in which it is stable without the need for an external force to hold it in that shape. The unit cell is formed using at least two different sections. One section is less pliable than the other one and acts a relatively rigid support that hinders the shape change of the more pliable section in one direction. In the other direction the pliable section can be deformed, but because of the opposing force from the rigid section, the stability of the pliable or flexible section is strongly increased.
External forces in a direction perpendicular to the most pliable section are distributed to the rigid section and the cross section of the pliable section is merely loaded in compression mode. This makes the construction much stronger than prior stents. In prior stents, all struts have generally the same cross section and mechanical properties and are merely used in the bending mode.
The construction of a stent, based upon this unit cell results in an apparatus, that can easily be elastically compressed around the balloon by finger pressure.
Below a certain critical diameter, the present stent snaps further to a stable, smallest diameter, thus holding the deflated balloon firmly on to the surface of the catheter, with an insertion diameter that is as small as possible. An additional sheath is not required, but may be used for extra safety.
After the stent has been brought into the patient""s body at the proper axial position, the balloon can be inflated until the stent reaches its critical elastic equilibrium diameter. Slightly above this diameter the stent automatically expands further to its final largest diameter, where it reaches its maximum stability against radial pressure. The design enables a constant length large expansion ratio, a reliable expandability and/or a small surface ratio.
A further embodiment of this invention is the possibility of a kind of stepwise expanding stent with a range of stable diameters.
Another part of the invention is a stent with several external diameters along its length, to adapt as good as possible to the shape of the body cavity where it is placed.
Another part of the invention is the possibility to modify the stress and strain pattern in the unit cell by means of a heat treatment in such a way, that the force displacement characteristic of this unit cell becomes asymmetrical or even exhibits a monostable instead of a bistable function, either with the expanded diameter or the collapsed diameter being the most stable condition.
Another embodiment of the invention is the modification of the geometry of the cross section of some struts to achieve the symmetric or asymmetric bistable or monostable force against displacement characteristics of a unit cell.
Another part of the invention is the use of one or more unit cells in other medical applications such as, for example, an expander or a clip, either to spread a body cavity open or to clamp or hold a body part or some body tissue.
The invention is also directed to the use of the inventive stents in conjunction with inventive expander rings to join together two vessels.
The invention is also directed to a bistable valve having a snap-action bipositional unit cell and uses for the same, in particular, to prevent incontinence.
The invention is also directed to multistable cells and their use in medical devices.
The construction of the present stent includes a series of elements with an arrangement of unit cells that enable the stability in a special way. Each unit cell exists out of at least two distinct, mechanically connected sections with different mechanical behaviors. One section acts as a relatively rigid support for the more flexible counteracting section. The more flexible section is responsible for most, if not all, of the expansion of the stent. There are several ways to manufacture a stent based upon this principle and it can be made from several materials, like polymers, composites, conventional metals or shape memory alloys with superelastic behavior or with temperature sensitive behavior.
It can be made from an arrangement of wire or strip, welded together at specific places. Another possibility is metal deposition in the desired pattern onto a substrate or the use of sintering of prealloyed powder.
A further method is making the stent from a tubular shaped starting material, with a pattern of slits or slots made in the wall by means of etching, grinding, cutting (e.g., with a laser, water, etc.), spark erosion or any other suitable method. The pattern can also be made in a flat plate and then welded, brazed or crimped to a more or less cylindrical shape or a cylindrical mid section with two conical ends with larger diameters.